Compaq 108164-003 - ProLiant - 800 Technical Manual page 13

I/O Performance Tuning of Compaq Servers
capable of transmitting 100 MB/s over tremendous distances. Fibre Channel can be deployed
using many topologies.
Fibre Channel Bandwidth & Saturation
Similar to the SCSI and PCI busses, a Fibre Channel Arbitrated Loop (FC-AL) can become
saturated and limit performance. Since FC bus can carry 100 MB/s, saturating the FC bus would
require at least two Fibre Channel Arrays, delivering greater than 50MB/s each. Compaq Fibre
Channel Arrays are dedicated storage enclosures with integrated SMART-2 technology, each
array can deliver a theoretical maximum of 80 MB/s. Here again, knowing the loads placed on
your server provides insight to the number of devices that would saturate your Fibre Channel.
In practice, only when operating in the most I/O stressful environments would the FC-AL become
saturated. For example, in order to saturate the FC loop, you would need to have three
simultaneous, broadcast-quality video streams on the FC. If you operate your server under such
extreme I/O conditions, you should install additional Fibre Channel Controllers to provide
another 100 MB/s pathway to your storage.
If you use multiple Fibre Channel loops, you should try to balance the I/O load between the
loops. However, balancing the loads may provide only marginal performance improvements
unless one loop has consistently high utilization.
Distance and Arbitration Issues
Compaq Fibre Channel technology allows greater distance than any other storage bus. However,
as the distances between server and storage, and storage and hub become several kilometers, the
delay introduced by the data traversing the bus becomes noticeable. The delay, caused by data
travelling at the speed of light, cannot be eliminated. By understanding the source of the delay,
you can better implement distant storage for your server.
Figure 4. Fibre Channel Arbitrated Loop Topology
ECG044.0399
Compaq Fibre
Channel Array 5
Fibre Channel
Storage Hub 7
Compaq Fibre
Channel Array 4
Compaq Fibre
Channel Array 3
Server
Compaq Fibre
Channel Array 0
Compaq Fibre
Channel Array 1
Compaq Fibre
Channel Array 2
13
10km
140km